In recent years, the GaN-based compound semiconductor materials have been attracting attention as semiconductor materials for use in the short wavelength light-emitting devices. The GaN-based compound semiconductors are formed on sapphire single crystals and various oxides and Group III-V compounds as substrates by the metal organic chemical vapor deposition method (MOCVD method), the molecular beam epitaxy method (MBE method), etc.
The GaN-based compound semiconductor materials have a characteristic feature of inducing small current diffusion in the lateral direction. Though the cause for this phenomenon has not been elucidated in detail, it may be probably ascribed to the presence of numerous dislocations threading the epitaxial crystal from the substrate through the first surface. Further, the p-type GaN compound semiconductor has high specific resistance as compared with the n-type GaN compound semiconductor and is not hardly enabled by simply stacking metal on the first surface to add to the lateral expanse of electric current in the p-layer and, when fabricated in an LED configuration having a p-n junction, is enabled to emit light only directly below the positive electrode.
Thus, it is common to use a transparent electrode as the p-electrode. For example, the idea of stacking Ni and Au on a p-layer and subjecting the stacked metals to an alloying treatment and consequently promoting decrease of the resistance of the p-layer and forming a positive electrode with transparent property and ohmic property, has been proposed (refer, for example, to Japanese Patent No. 2804742).
For the purpose of acquiring bonding strength in the pad electrode, a structure which is enabled, by cutting off a portion of a transparent electrode and forming a pad electrode throughout on the transparent electrode and a straddle the cut-off portion, to acquire the bonding strength in the part directly contiguous to the GaN layer and at the same time attain current diffusion in the part contiguous to the transparent electrode, has been laid open to public inspection (refer, for example, to JP-A HEI 7-94782).
Because a given metal ideally acquires ohmic contact, it does not necessarily follow that this metal shows a high mechanical-contact-strength. When a bonding pad is allowed to contact a semiconductor layer, the contact entails the problem that the part of this contact inevitably gives rise to an increase in the contact resistance and consequently suffers the forward voltage (VF) to rise.
In short, the bonding pad is effective in lowering the operation voltage in the forward direction when the area of contact which it produces with the semiconductor layer is decreased.
This invention has for an object the provision of a transparent electrode for a gallium nitride-based compound semiconductor light-emitting device, which transparent electrode produces excellent ohmic contact and current diffusion and abounds in contact strength of bonding pad as well. The term “transparent property” as used in this invention means that the pertinent electrode is transparent to the light of a wavelength in the range of 300 to 600 nm.